The results of the current study demonstrate a strong response to temperature with increasing vegetative development time at progressively lower temperatures in both cultivars, as well as a strong influence on shoot and tuber biomass per plant in both cultivars. As previously observed in pot-experiments (Johansen and Mølmann 2017, 2018), Gullauge emerged from soil earlier than Mandel, and this difference was maintained throughout the vegetative development of leaves (BBCH10-19) and visible flower buds (BBCH55). Van Delden et al. (2020) have also reported that only a small cultivar difference in emergence thermal time, with subsequent significant difference in leaf area index between the cultivars Bintje and Eersteling. There was also a clear difference between cultivars with higher plant height and higher tuber yields in Gullauge than in Mandel. Our previous studies on these cultivars have also shown higher plant height and tuber yield for Gullauge, when grown under field conditions in Tromsø and in pot-experiments in greenhouse (Johansen and Mølmann 2017, 2018). This maybe in part be due to the earlier emergence in Gullauge compared to Mandel, and the lower development rates seen for Mandel in the current experiment. Furthermore, Gullauge produced more tubers per plant, also adding at least partly to the higher yields in Gullauge compared to Mandel.
The observed gradual increase in plant height with corresponding increases in temperature in both cultivars, was probably due to increased stem biomass as the measured total shoot DM content (including stems and leaves) seemed to plateau at the intermediate temperatures 12 and 15°C in both cultivars. This agrees with phytotron experiments with the cultivars Russet Burbank and Norland, producing a model with an apparent steady linear increase in stem weight across tested temperatures ranging from 16 to 24°C (Yandell et al. 1988). More recent studies involving microtubers and true seedlings have also demonstrated a positive response in stem elongation to elevated temperature (Kacheyo et al. 2024; Lavrynenko et al. 2016). However, in contrast to the current study, our previous field trials had no discernable difference in shoot biomass between Gullauge and Mandel at harvest in autumn, and lower tuber DM percent in Gullauge than Mandel (Johansen and Mølmann 2017, 2018). In the current study we observed higher shoot biomass and lower tuber DM percent for Mandel for Gullauge. This may be due to differences in growing conditions inside a greenhouse with solar radiation through glass and constant temperatures, as opposed to higher solar irradiance outside and fluctuating diurnal and seasonal temperatures. Light intensity and diurnal temperature difference both have a positive effect on tuber numbers and yield per plant in potato tuber seedlings (Stockem et al. 2023). In addition, in temperature regulated climate chamber there is little to no difference between soil and air temperature, as compared to field studies with often differences in soil and air temperature depending on soil humidity and the balance between solar irradiance and heat radiation from the ground or canopy. Slightly lower soil to air temperature is positive for development of dry matter content in potato tubers (Epstein 1966).
The current results agree with previous studies showing higher optimum temperatures for emergence and above-ground growth and development, than for below-ground tuber development and yields. The development rate for emergence, leaf formation and flowering indicated optimum temperatures for both Gullauge and Mandel above 21°C, the highest temperature used in the experiment. The respective estimated optimums at 24.0°C and 22.6°C are within range of reported optimums for vegetative development in controlled climate studies. The temperature optimum for sprout growth on seed tubers was between 20–22°C for early and semi-late cultivars (Klemke and Moll 1990). The optimum temperature for subsequent leaf appearance rate, similar to our observed data for BBCH11 to BBCH19, have been modeled to be 27.2°C for Kennebec cultivar (Fleisher et al. 2006). Tuber DM percentage may be used as an indicator of tuber development/maturity, and the estimated optimum for tuber DM percent at 16.6°C for Gullauge and 15.6°C for Mandel are close in range of results from previous phytotron studies. The cultivars Russet Burbank and Norland have been modeled with optimums of 17.5 and 18.7 respectively (Yandell et al. 1988). However, since vegetative development was about a week earlier for our two warmest temperature treatments, it possible that tuberization started earlier for these and that temperature optimums for tuber development per se are lower than our estimates. The estimated optimums for yields are indeed lower than for DM percent, at 14.9°C and 14.6°C respectively for Gullauge and Mandel. The intensity of red pigment in parts of the skin and eyes of the cultivar Gullauge has been suggested to be a response of maturity. However, our results with increasing red areas in the skin at increasing temperatures up to 21°C and an optimum for tuber development at 16.6°C, could indicate that this is rather a response to temperature. Cultivars of red and violet skin colour do seem to develop more anthocyanins at intermediate and warm temperatures (Bethke & Jansky, 2021; Chen et al. 2024), with slightly decreasing concentrations as the tubers swell towards maturity (Reyes et al. 2004; Šulc et al. 2017).
From a practical perspective, temperature optimums may be considered the temperature at which photosynthesis contributes 100% to the net growth of the plant or plant part in question. The current study was performed in 24 h light under Midnight Sun above the Arctic Circle, in very long daily photosynthesis periods. Interestingly, growing potato under very long daily photosynthetic light periods appears to reduce the optimum temperature for both tuber and shoot DM yield, compared to growth in short photoperiod. For example, a phytotron experiment with the cultivar Norland had highest yields at temperatures 4 degrees lower in 24 h photoperiod, than in 12 h photoperiod (Wheeler et al. 1986). Similarly, another study with cv. Spunta and Désirée also displayed highest shoot yields in treatments four degrees lower at 18 h compared to a short 12 h photoperiod (VanDam et al. 1996). Both these studies also appeared to have higher optimums for leaf and stem yield, compared to lower optimums for tuber yields (Fleisher et al. 2006; Yandell et al. 1988). Since the mean daily photosynthetic light period in the current experiment period was measured to be around 18 h, this suggests that optimum temperatures for Gullauge and Mandel may be higher if grown in shorter photoperiod at lower latitudes or in late autumn outside the main growing season when days are shortening. Developing local-adapted growth models therefore also needs to consider the light conditions during the growth season where the model will be used.
In conclusion, we observed a strong response to temperature in both cultivars, with higher temperature optimum for above-ground shoot development than below-ground tuber development. The experimental setup with constant temperatures under steady airflow in climate-controlled growth chambers ensures the same temperature in soil and at leaf-level in shoots, enabling the separation of temperature optimums for both above- and below-ground development. Different temperature optimums for shoot and tuber development may be utilized in a combined growth model, considering both emergence and shoot growth, and below- ground tuber growth and development. This of course needs to be further explored under variable climatic conditions outside in production fields to adjust for the relative importance or timing of model parameters.